In the art of manufacturing pneumatic tires, rubber flow in the mold or minor differences in the dimensions of the belts, beads, liners, treads, plies of rubberized cords, sometimes cause non-uniformities in the final tire. When non-uniformities are of sufficient magnitude, they will cause force variations on a surface, such as a road, against which the tires roll and thereby produce vibrational and acoustical disturbances in the vehicle upon which the tires are mounted. Regardless of the cause of the force variations, when such variations exceed the acceptable minimum level, the ride of a vehicle utilizing such tires will be adversely affected.
The effects of non-uniformity are best explained by noting that several types of forces, which are of particular importance in the present application, are simultaneously exerted by a tire during its rotation under load against a surface. For example, radial run-out is an intrinsic tire non-uniformity best described as the "out of roundness" of the tire. Also radial forces are exerted in the radial direction of the tire, or in a direction perpendicular to its axis of rotation and non-tangential to the road surface. Additionally, lateral forces are exerted in the axial direction of the tire or in a direction parallel to its axis of rotation. Further, excessive conicity, defined as one-half of the net average lateral force resulting from a non-conical shaped tire, causes a tire to constantly pull to one side.
In a non-uniform tire, the radial run-out, the radial forces, and the lateral forces exerted by the tire will vary or change during its rotation. In other words, the magnitude and/or direction of the radial run-out, and the radial and lateral forces exerted by the tire will depend on which increment of its tread is contacting the surface.
The variations in radial and lateral force during rotation of a tire are usually caused by differences in the stiffness and/or geometry of the tire about its circumference or tread centerline. If these differences are slight, the radial and lateral force variations and therefore the degree of conicity will be insignificant and their effects unnoticeable when the tire is installed on a vehicle. However, when these differences reach a certain level, the radial and/or lateral force variations may be significant enough to cause rough riding conditions and/or difficult handling situations. Also, an excessive conicity value will cause a rolling tire to pull to one side.
Consequently, methods have been developed in the past to correct for excessive force variations by removing rubber from the shoulders and/or the central region of the tire tread by means such as grinding. Most of these correction methods include the steps of indexing the tire tread into a series of circumferential increments and obtaining a series of force measurements representative of the force exerted by the tire as these increments contact a surface. This data is then interpreted and rubber is removed from the tire tread in a pattern related to this interpretation. These methods are commonly performed with a force variation machine which includes an assembly for rotating a test tire against the surface of a freely rotating loading drum. This arrangement results in the loading drum being moved in a manner dependent on the forces exerted by the rotating tire whereby forces may be measured by appropriately placed measuring devices. In a sophisticated force variation machine (FVM), such as a Model No. D70LTW available from the Akron Standard Co. of Akron Ohio. The force measurements are interpreted by a computer and rubber is removed from the tire tread by grinders controlled by the computer. Examples of these methods are disclosed for example in U.S. Pat. Nos. 3,739,533, 3,946,527, 4,914,869, and 5,263,284.
As illustrated by prior patents and commercial devices, as described above, efforts are continuously being made to more efficiently correct tire non-uniformity. None of these prior art efforts, however, suggest the present inventive combination of method steps and component elements arranged and configured for correcting the conicity parameter, as well as the order of the routine for correcting variations in lateral forces, followed by radial run-out and finally radial forces as disclosed and claimed herein. Prior methods and apparatus do not provide the benefits of the present invention which achieves its intended purposes, objectives and advantages over the prior art devices through a new, useful and unobvious combination of method steps and component elements, through no increase in the number of functioning parts, at a reduction in operational cost, and through the utilization of only readily available materials and conventional components.
It is an object of the present invention to provide a method for correcting or shifting the conicity value in a pneumatic tire to obviate the problems and limitations of the prior art methods. Other objects of this invention will be apparent from the following description and claims.